An X-ray generator is generally known as a device to generate X-rays to irradiate the diagnostic region of the body of a subject, and is comprised of an X-ray tube which irradiates X-rays and a high-voltage generator which generates high-voltage DC (hereinafter referred to as the tube voltage) to apply to said X-ray tube. The neutral grounded type has mainly been used for the stated X-ray generator. However, it has been difficult to accommodate the centrifugal force-resistant capacity in the anode roller bearing portion in the cases of achieving the anode heat capacity or adapting it to a CT device. Consequently an anode grounded X-ray tube has started to be used as well, in accordance with the increase in capacity and load factor of the X-ray generator as disclosed in JP-A-2002-164197. This anode grounded X-ray tube is configured in a way that the electric potential of an anode rotating rotor can be grounded, which increases the degree of freedom in designing the anode, making it possible to facilitate the designing for heat release, allowing for dramatically improved heat release efficiency. The mounting of a large number of X-ray tubes became possible as a result.
Patent Document 1: JP-A-2002-164197
However, using the inverter type high-voltage generator with the conventional anode grounded X-ray tube leaves us with no choice but to enlarge the size of its housing in order to withstand the voltage. The high-voltage generator with conventional anode grounded type was configured to hold the DC voltage of +75 kV maximum for the anode side, −75 kV maximum for the cathode side, with a total of 150 kV to be applied to an X-ray tube in response to the earth potential, thus required the designing to withstand ±75 kV maximum for the windings of a high-voltage transformer or for between the respective terminals and the earth potential of a high-voltage rectifier. On the other hand, in the case of using the anode grounded X-ray tube, the cathode side requires a maximum of −150 kV for grounding the anode side of the X-ray tube in response to the earth. Consequently, a design to withstand two times 75 kV is demanded, and the size of a high-voltage generator including a high-voltage transformer for an anode grounded X-ray tube or a high-voltage rectifier would have to be quite large.
Meanwhile in another document, Japanese Patent No. 2814016, the Cockcroft-Walton circuit is disclosed as a voltage multiplying circuit. The operation of the above-mentioned circuit will now be described using FIG. 3 of the above-mentioned document.
Patent Document 2: Japanese Patent No. 2814016    (1) In a cycle of the secondary coil in which the upper side becomes a positive, an electric current flows through diode 19, passing through capacitor 17 from above the second coil. At this time, the voltage −E(kV) which is at peak alternating voltage will be charged at both ends of capacitor 17.    (2) Next, in a cycle in which the polarity of alternating voltage reverses its course and the underside of the secondary coil turns to be a positive, a secondary current flows toward capacitor 21 from underneath. Capacitor 21 is charged by −E (kV), and the electric current returns to the secondary coil, passing through diode 18 and capacitor 17. At this time, the electric current, passing through diode 18, is backed up by −E(kV) which was maintained within said capacitor 17. As a result, −2E(kV), a total of −E(kV) which was generated in the secondary coil and the voltage −E(kV) which was in capacitor 17, is generated between both ends of capacitor 21.    (3) Moreover, in a cycle in which the polarity of alternating current is reversed and the upper side of the secondary coil becomes a positive again, an electric current flows in a same manner as (1), and −E(kV) in capacitor 17 which started to fall will be maintained.    (4) Moreover, in a cycle in which the polarity of the alternating current reverses and the underside of the secondary coil becomes a positive again, −2E(kV) is generated totaling −E(kV) which was generated in the secondary coil and −E(kV) which was stored in capacitor 17 as described in (3) between the earth and the upper side of the secondary coil. At this point −2E(kV) has already been generated as described in (2) at both ends of capacitor 21. In this way the cathode potential of the X-ray tube is stabilized at −2E(kV). Furthermore, on and after (2), electricity is constantly discharged by the X-ray tube after the voltage received at both ends of the X-ray tube reaches a certain level. The voltage that capacitor 21 receives is generally around −150 kV at this point, which requires capacitor 21 to be quite large in size. Additionally, notable ripples are included in the voltage drop curved line on both ends of the X-ray tube at the time of discharge.
The purpose of this invention is to offer an inverter type X-ray generator that allows for small-size and light-in-weight configuration at a reduced cost even with usage of the anode grounded X-ray tube operated with high voltage, and is able to reduce the ripples during discharge.